Two Constrating Garnet Peridotites And Garnet Pyroxenites In The South Altyn Tagh-North Qaidam HP/UHP Orogenic Belt

The South Altyn Tagh-North Qaidam HP/UHP belt,which is located at the northern margin of Tibet Plateau,is characterized by the occurrence of eclogite,garnet peridotite and related gneiss.They occur as slices or lenses within felsic geniss with garnet pyroxenite and granulites.On the basis of textural relationships,mineralogical data and thermobarometer estimations,four stages of the evolution have been identified for the Bashiwake and Lvliangshan garnet peridotites,including the protolith stage(M1),the peak metamorphic stage(M2),the decompression retrograde stage(M3)and the late amphibolite-chlorite-facies retrograde stage(M4).Stage I is represented by the inclusions(e.g.spinel,olivine,clinopyroxene)in the garnet;Stage ? is defined by the balanced assemblage of large porphyroblasts occurred in the matrix;the granulite-facies assemblage of stage ? is manifested as kelyphitic development around coarse-grained garnet porphyroblasts or the symplektite resulted from some decompression reaction.The final(amphibole-greenschist-facies)retrogressive stage IV is characterized by recrystallization of amphibole(Amp)+ serpentine(Srp)+ phlogopite(Phl)+ chlorite(Chl)+ magnetite(Mag)± talc.The yielded P-T conditions of peak matemorphism for the Bashiwake and Llviangshan garnet peridotite are 890~1050 °C,24.2~30.3 kbar and 870~940 °C,43.5~51.7 kbar,respectively.Three stages of the metamorphic evolution have been identified for the Bashiwake garnet pyroxenite,including the peak metamorphic stage(M1),the decompression retrograde stage(M2)and the late amphibolite-chlorite-facies retrograde stage(M3).Stage I is represented by the balanced assemblage of garnet,clinopyroxene,kynite,rutile and quartz;The granulite-facies assemblage of stage ? is manifested as the symplektite resulted from some decompression reaction.The final(amphibole-greenschist-facies)retrogressive stage ? is characterized by recrystallization of amphibole(Amp)+chlorite(Chl)± talc.By constrst,the Lvliangshan garnet pyroxenites also have three stages of the metamorphic evolution,and the principle is the same as that of the Bashiwake garnet pyroxenite excepted the mineral assemblage.The yielded P-T conditions of peak matemorphism for the Bashiwake and Llviangshan garnet pyroxenite are 860~950 °C,22.5~26.7 kbar and 772~881°C,28.4~39.7 kbar,respectively.Geochronology of zircon U-Pb indicates that the peak metamprphic age for the Bahiwake garnet peridotite and garnet pyroxenite are ~500Ma,which is in consistent with the time of eclogite-facies UHP metamorphism of adjacent eclogite unit,and the age of the protolith are ~800Ma for the Bahiwake garnet peridotite and garnet pyroxenite.Combining with mineralogical and geochemical characteristics,we speculated that the protolith of garnet peridotite was mantle-derived mafic-ultramafic complex emplaced in the crust,which was subducted together with felsic crust material in the early Paleozoic,and they shared the subsequent metamorphic and geodynamic evolution.Geochronology of zircon U-Pb indicates that the peak metamprphic age for the Lvliangshan garnet peridotite and garnet pyroxenite are not late ~430Ma,and the Lvliangshan peridotite in the Qaidam terrane is a fragment mantle wedge beneath the Qilian crust,which was exhumed together with the previously subducted Qaidam continental crust.The overlying mantle wedge was metasomated by the addition of fluids and/or melts.Although the Bashiwake and Lvliangshan peridotite of South Altyn Tagh-North Qaidam HP/UHP terrane possess significantly different metamorphic age,initial tectonic setting and P-T-t path,they were the product the diachronous or protracted metamorphism related to the subdution.These two types of peridotites are similar to the type A and type B peridotites within the Sulu-Dabie orogenic belt.Comparing these rocks from different initial settings is to reveal the mantle material composition and the dynamics process of South Altyn Tagh-North Qaidam HP/UHP terrane.